This invention relates to a heat exchanger for a combustion apparatus, and more particularly to a heat exchanger for a combustion apparatus which is adapted to heat an air-pervious regenerator by exhaust gas and then preheat an oxidizing agent for combustion by the thus-heated air-pervious regenerator, followed by feeding of the oxidizing agent thus preheated to a combustion chamber.
A heat recovery type combustion apparatus in which a conventional rotary heat exchanger is incorporated is disclosed by way of example in Japanese Patent Application Laid-Open Publication No. 222102/1989. The conventional rotary heat exchanger disclosed is constructed so as to rotate an air-pervious regenerative member or regenerator made of a ceramic material with respect to a suction/exhaust duct structure, to thereby carry out heat exchange. Also, the conventional heat exchanger is constructed so as to provide an air passage for guiding air acting as an oxidizing agent to a combustion chamber in an exhaust passage for guiding exhaust gas discharged from the combustion chamber and terminate a part of a partition for partitioning the air passage and exhaust passage from each other at a position apart from the air-pervious regenerator, to thereby permit the air passage and exhaust passage to partially communicate with each other. Unfortunately, partial communication between the air passage and the exhaust passage causes fluid to flow from a high-pressure fluid side through a communication section therebetween into a low-pressure fluid side, so that the heat exchanger requires to employ a forced draft fan for feeding combustion air to an air duct and an induced-draft fan for drawing exhaust gas out of an exhaust duct which are increased in capacity in view of such flowing of fluid into the low-pressure fluid side. Also, this fails to permit the heat exchanger to be increased in heat transfer efficiency and heat recovery, leading to a failure to satisfactorily increase energy saving of the heat exchanger. In view of the problem, the assignee proposed techniques wherein a seal structure is provided between an open end of an air duct and an air-pervious regenerator to minimize leakage of combustion air from an air passage to an exhaust passage, to thereby permit both a forced draft fan for feeding of combustion air and an induced-draft fan for suction of exhaust gas to be decreased in capacity.
Such a conventional rotary heat exchanger as described above is likewise disclosed in Japanese Patent Application Laid-Open Publication No. 241436/1994, U.S. Pat. No. 5,275,556, U.K. Patent Application Publication No. 2,208,423 and the like.
Another conventional heat exchanger for a combustion apparatus is proposed which is constructed so as to heat an air-pervious generator by means of exhaust gas and then preheat air by the regenerator thus heated while keeping the regenerator and a duct from being rotated, as disclosed in Japanese Patent Application Laid-Open Publication No. 256423/1993, Japanese Patent Application Laid-Open Publication No. 11121/1994, U.S. Pat. No. 4,865,492 and the like. Also, the heat exchanger proposed is constructed in such a manner that the air-pervious regenerator is arranged between a combustion chamber and each of two suction/exhaust ducts or in the middle of each of the suction/exhaust ducts. Also, at least one reversing valve is provided for the two suction and exhaust ducts. Changing-over or reversing of the reversing valve permits the two suction/exhaust ducts to alternately function as an air duct and an exhaust duct, respectively.
Of the conventional heat exchangers described above, the former fails to provide satisfactory sealing between the open end of the air duct and the air-pervious regenerator irrespective of arrangement of the seal structure therebetween, because the air-pervious regenerator is formed with a number of through-holes.
The latter would be improved in sealing characteristics as compared with the former. Nevertheless, in the latter, it was found that leakage of air occurs in the reversing valve kept closed. FIG. 8 schematically shows reversing of the two suction/exhaust ducts by means of a four-direction reversing valve, wherein reference numeral 101 designates a combustion chamber, 102 and 103 each are a duct, 104 and 105 each are an air-pervious regenerator, and 106 is a four-direction reversing valve. When a valve body 107 of the four-direction reversing valve 106 is rotated by an angle of 90 degrees in a clockwise direction from a position shown in FIG. 8, the ducts 102 and 103 are permitted to function as an air duct and an exhaust duct, respectively. For example, flowing of air through the duct 102 causes a low-temperature end difference pressure which is a difference in pressure between an air pressure on an inlet side of the air-pervious regenerator 104 and an exhaust pressure on an outlet side of the sir-pervious regenerator 105 to be applied to the valve body 107. Thus, in order to actuate or operate the valve body 107, driving force is required which is proportional to a product of the low-temperature end difference pressure and a flow rate of fluid flowing through the valve. However, the reversing valve is reversed many times during operation of the combustion apparatus, so that even a slight decrease in driving force for the valve body 107 leads to a reduction in cost for operation of the heat exchanger and an improvement in durability of the reversing valve. Also, use of the reversing valve generally comprising a butterfly valve fails to fully prevent air leakage, which is proportional to a root of the low-temperature end differential pressure. In order to avoid a decrease in thermal recovery and effectiveness of the heat exchanger, it is required to increase the low-temperature end differential pressure. Unfortunately, an increase in low-temperature end differential pressure causes air leakage to be increased, so that it is required to excessively large-size the forced and inducted-draft fan and ducts to be incorporated in the heat exchanger.